Method and apparatus for intra prediction

ABSTRACT

Disclosed herein are a method and apparatus for filtering a reference pixel. A method of smoothing a reference pixel may include determining whether or not to perform smoothing on a reference pixel based on the size of a transform block and information about an intra-prediction mode for the transform block and determining a smoothing method by comparing the size of the transform block with the size of a maximum transform block. Accordingly, image coding efficiency can be increased, and picture quality can be improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/477,352 filed on Apr. 3, 2017, which is a continuation of U.S. patentapplication Ser. No. 14/173,992 filed on Feb. 6, 2014 and issued as U.S.Pat. No. 9,615,086, which claims the benefit under 35 USC 119(a) ofKorean Patent Application No. 10-2013-0024988 filed on Mar. 8, 2013 andKorean Patent Application No. 10-2013-0013547 filed on Feb. 6, 2013, theentire disclosures of which are incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an image encoding/decoding method andapparatus and, more particularly, to an intra-prediction method.

Discussion of the Related Art

Recently, a demand for images having high resolution and high quality,such as High Definition (HD) images and Ultra High Definition (UHD)images, is increasing in various application fields. As image databecomes high resolution and high quality, the amount of data isrelatively more increased than existing image data. If image data istransmitted using an existing wired/wireless broadband line or storedusing an existing storage medium, transmission costs and storage costsare increased. In order to solve the problems occurring due to the highresolution and high quality of image data, high-efficiency imagecompression techniques can be used.

The image compression techniques may include various techniques, such asan inter-prediction technique for predicting a pixel value included in acurrent picture in pictures anterior or posterior to the currentpicture, an intra-prediction technique for predicting a pixel valueincluded in a current picture using information about pixels within thecurrent picture, and an entropy coding technique for assigning a shortsymbol to a value having a high frequency of appearance and a longsymbol to a value having a low frequency of appearance. Image data canbe effectively compressed, transmitted, or stored using such imagecompression techniques.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an intra-predictionmethod.

Another object of the present invention is to provide an apparatus forperforming an intra-prediction method.

In accordance with an aspect of the present invention, anintra-prediction method may include determining whether or not toperform smoothing on a reference pixel based on the size of a transformblock and information about an intra-prediction mode for the transformblock and determining a smoothing method by comparing the size of thetransform block with the size of a maximum transform block. Thedetermining of whether or not to perform smoothing on the referencepixel based on the size of the transform block and the information aboutthe intra-prediction mode for the transform block may includedetermining whether or not the size of the transform block is 8×8 andthe intra-prediction mode for the transform block is one of a No. 2intra-prediction mode, a No. 18 intra-prediction mode, a No. 34intra-prediction mode, and a planar mode, determining whether or not thesize of the transform block is 16×16 and the intra-prediction mode forthe transform block is one of a No. 2 intra-prediction mode to a No. 8intra-prediction mode, a No. 12 intra-prediction mode to a No. 24intra-prediction mode, a No. 28 intra-prediction mode to a No. 34intra-prediction mode, and a planar mode, and determining whether or notthe size of the transform block is 32×32 and the intra-prediction modefor the transform block is one of a No. 2 intra-prediction mode to a No.34 intra-prediction mode and a planar mode other than a No. 10intra-prediction mode and a No. 26 intra-prediction mode. Thedetermining of the smoothing method by comparing the size of thetransform block with the size of the maximum transform block may includedetermining a bi-linear smoothing filter to be the smoothing filter if,as a result of the comparison, the size of the transform block isidentical with the size of the maximum transform block and determining asmoothing filter having filtering coefficients of [1,2,1] to be thesmoothing filter if, as a result of the comparison, the size of thetransform block is not identical with the size of the maximum transformblock. The intra-prediction method may further include decoding flaginformation, wherein the flag information may be information fordetermining whether or not to perform smoothing on the reference pixelby determining whether or not to perform smoothing on the referencepixel based on the size of the transform block and the information aboutthe intra-prediction mode for the transform block and determining thesmoothing method by comparing the size of the transform block with thesize of the maximum transform block. The intra-prediction method mayfurther include determining an intra-prediction method for a specificintra-prediction mode by comparing the size of the transform block withthe size of the maximum transform block.

In accordance with another aspect of the present invention, an imagedecoder may include an intra-prediction unit, wherein theintra-prediction unit may determine whether or not to perform smoothingon a reference pixel based on the size of a transform block andinformation about an intra-prediction mode for the transform block anddetermine a smoothing method by comparing the size of the transformblock with the size of a maximum transform block. The intra-predictionunit may be configured to determine whether or not the size of thetransform block is 8×8 and the intra-prediction mode for the transformblock is one of a No. 2 intra-prediction mode, a No. 18 intra-predictionmode, a No. 34 intra-prediction mode, and a planar mode, determinewhether or not the size of the transform block is 16×16 and theintra-prediction mode for the transform block is one of a No. 2intra-prediction mode to a No. 8 intra-prediction mode, a No. 12intra-prediction mode to a No. 24 intra-prediction mode, a No. 28intra-prediction mode to a No. 34 intra-prediction mode, and a planarmode, and determine whether or not the size of the transform block is32×32 and the intra-prediction mode for the transform block is one of aNo. 2 intra-prediction mode to a No. 34 intra-prediction mode and aplanar mode other than a No. 10 intra-prediction mode and a No. 26intra-prediction mode. The intra-prediction unit may be configured todetermine a bi-linear smoothing filter to be the smoothing filter if, asa result of the comparison, the size of the transform block is identicalwith the size of the maximum transform block and determine a smoothingfilter having filtering coefficients of [1,2,1] to be the smoothingfilter if, as a result of the comparison, the size of the transformblock is not identical with the size of the maximum transform block. Theimage decoder may further include an entropy decoding unit. The entropydecoding unit may be configured to decode flag information, and the flaginformation may be information for determining whether or not to performsmoothing on the reference pixel by determining whether or not toperform smoothing on the reference pixel based on the size of thetransform block and the information about the intra-prediction mode forthe transform block and determining the smoothing method by comparingthe size of the transform block with the size of the maximum transformblock. The intra-prediction unit may be configured to determine anintra-prediction method for a specific intra-prediction mode bycomparing the size of the transform block with the size of the maximumtransform block.

Image coding efficiency can be increased and picture quality can beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram showing a coder in accordance with anembodiment of the present invention;

FIG. 2 is a block diagram of a decoder in accordance with an embodimentof the present invention;

FIG. 3 is a conceptual diagram showing a smoothing filter applicationmethod for a reference pixel in accordance with an embodiment of thepresent invention;

FIG. 4 is a conceptual diagram showing a reference pixel smoothingmethod using a smoothing filter in accordance with an embodiment of thepresent invention;

FIG. 5 is a conceptual diagram showing a reference pixel smoothingmethod using a smoothing filter in accordance with an embodiment of thepresent;

FIG. 6 is a flowchart illustrating a method of coding information fordetermining whether or not to perform smoothing on a reference pixel inaccordance with an embodiment of the present invention; and

FIG. 7 is a flowchart illustrating a method of decoding information fordetermining whether or not to perform smoothing on a reference pixel inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Elements described in the embodiments and drawings of the presentinvention have been disclosed as independent elements in order toindicate different and characteristic functions of an image coder. Itdoes not necessarily mean that each of the elements is formed of a pieceof separated hardware or a piece of software. That is, the elements arearranged and included, for convenience of description, and at least twoof the elements may be combined to form one element or one element maybe divided into a plurality of elements that perform functions. Anembodiment in which some elements are integrated or embodiments in whichsome elements are separated are also included in the scope of thepresent invention unless they depart from the essence of the presentinvention.

Furthermore, some elements disclosed in the present invention are notessential elements for performing essential functions, but may beoptional elements for improving only performance. The present inventionmay be implemented using only elements essential for implementing theessence of the present invention other than elements used to improveonly performance, and a structure including only essential elementsother than optional elements used to improve only performance is alsoincluded in the scope of the present invention.

FIG. 1 is a conceptual diagram showing a coder in accordance with anembodiment of the present invention.

Referring to FIG. 1, the coder may include a partition unit 100, aprediction unit 110, an intra-prediction unit 103, an inter-predictionunit 106, a transform unit 115, a quantization unit 120, a rearrangementunit 125, an entropy coding unit 130, an inverse quantization unit 135,an inverse transform unit 140, a filter unit 145, and memory 150.

The coder may be implemented according to an image coding methoddescribed below in connection with an embodiment of the presentinvention, but an operation in some elements may not be performed inorder to reduce the complexity of a coder or for fast real-time coding.For example, when the prediction unit performs intra-prediction, amethod of using a limited number of intra-prediction modes and selectingone of the intra-prediction modes as the final intra-prediction mode,instead of a method of using all intra-prediction mode methods andselecting an optimum intra-coding method from all the intra-predictionmode methods, may be used in order to perform coding in real time. Foranother example, when performing intra-prediction or inter-prediction, aform of a prediction block used may be limitedly used.

The unit of a block processed by the coder may be a coding unit on whichcoding is performed, a prediction unit on which prediction is performed,or a transform unit on which transform is performed. The coding unit mayalso be represented as a term ‘CU’, the prediction unit may also berepresented as a term ‘PU’, and the transform unit may also berepresented as a term ‘TU’. Alternatively, the coding unit may also berepresented as a term ‘coding block’, the prediction unit may also berepresented as a term ‘prediction block’, and the transform unit mayalso be represented as a term ‘transform block’.

The partition unit 100 can partition one picture into combinations of aplurality of coding blocks, prediction blocks, and transform blocks,select a combination of one coding block, prediction block, andtransform block as a specific criterion (e.g., a cost function), andpartition a picture using the selected combination. For example, inorder to partition a coding block in a picture, a recurrence treestructure, such as a quad tree structure, may be used. Hereinafter, inan embodiment of the present invention, a coding block may mean a blockto be decoded as well as a block to be coded.

A prediction block may become a unit on which intra-prediction orinter-prediction is performed. A block on which intra-prediction isperformed may have a square form, such as 2N×2N or N×N, or a rectangleform using Short Distance Intra-Prediction (SDIP). A block on whichinter-prediction is performed may have a square form, such as 2N×2N orN×N, or may include a prediction block partition method using AsymmetricMotion Partitioning (AMP), that is, 2N×N, N×2N, or an asymmetrical formin which a prediction block having a square form is bisected in the sameform. A transform method performed by the transform unit 115 may bedifferent depending on the type of prediction block.

The prediction unit 110 may include the intra-prediction unit 103 forperforming intra-prediction and the inter-prediction unit 106 forperforming inter-prediction. The prediction unit 110 can determinewhether or not to use inter-prediction for a prediction block or whetheror not to perform intra-prediction on a prediction block. A processingunit on which prediction is performed may be different from a unit of aprocessing block in which a prediction method is determined. Forexample, in performing intra-prediction, a prediction mode may bedetermined based on a prediction block, and a process of performingprediction may be performed based on a transform block. A residual value(or a residual block) between a generated prediction block and theoriginal block can be inputted to the transform unit 115. Furthermore,information about a prediction mode and information about a motionvector that are used for prediction, together with the residual value,can be coded by the entropy coding unit 130 and transferred to adecoder.

If a Pulse Coded Modulation (PCM) coding mode is used, prediction maynot be performed through the prediction unit 110, but the original blockmay be coded without change and transmitted to the decoder.

The intra-prediction unit 103 can generate an intra-predicted blockbased on reference pixels that are present in the periphery of a currentblock (i.e., a block to be predicted). In order to estimate an optimumintra-prediction mode for the current block, an intra-predicted blockfor the current block can be generated using a plurality ofintra-prediction modes. The intra-prediction unit 103 can select apredicted block using one intra-prediction mode and uses the selectedpredicted block as a predicted block for the current block. In anintra-prediction method, an intra-prediction mode may include adirectional prediction mode in which information about reference pixelsis used depending on a prediction direction and a non-directional mode(or a DC mode or planar mode) in which directional information is notused when performing prediction. The type of mode for predicting lumainformation may be different from the type of mode for predicting chromainformation. In order to predict chroma information, information aboutan intra-prediction mode predicted from luma information or informationabout a predicted luma signal may be used.

Regarding a current block on which intra-prediction has been determinedto be performed using one intra-prediction mode, an intra-predictionmode for the current block can be predicted from information about anintra-prediction mode that has been used by the neighboring blocks ofthe current block when performing intra-prediction, and the informationabout the intra-prediction mode of the current block can be coded. Thatis, the intra-prediction mode for the current block can be predictedfrom intra-prediction modes for prediction blocks that are present inthe periphery of the current block. The following methods may be used asa method of predicting an intra-prediction mode for a current blockusing information about modes that have been predicted from neighboringblocks.

1) If an intra-prediction mode for a current block is the same as anintra-prediction mode for neighboring blocks, specific flag informationmay be coded, and information indicating that the intra-prediction modefor the current block is the same as the intra-prediction mode for theneighboring blocks may be transmitted.

2) If an intra-prediction mode for a current block is different from anintra-prediction mode for neighboring blocks, information about theintra-prediction mode for the current block may be subject to entropycoding, and information about a prediction mode for the current blockmay be coded.

An intra-prediction mode for neighboring blocks used to code anintra-prediction mode for a current block in the methods 1) and 2) maybe defined as a candidate intra-prediction mode and used.

If an intra-prediction mode for neighboring blocks is not available inperforming the methods 1) and 2) (e.g., the neighboring blocks are notpresent or inter-prediction has been performed on the neighboringblocks), a predetermined and specific intra-prediction mode value may beset as a candidate intra-prediction mode value, and an intra-predictionmode for a current block may be predicted.

The intra-prediction unit 103 can generate an intra-predicted blockbased on information about reference pixels in the periphery of acurrent block, that is, information about pixels within the currentpicture. If a block neighboring a current block is a block on whichinter-prediction has been performed, a reference pixel may be a pixelreconstructed from a pixel that has been predicted by performinginter-prediction. In such a case, the current block may be subject tointra-prediction using the pixels of the neighboring block on whichintra-prediction has been performed as reference pixels without usingthe reconstructed pixel. That is, if a reference pixel is not available,the unavailable reference pixel may be replaced with another pixel.

A prediction block may include a plurality of transform blocks. If thesize of a prediction block is the same as the size of a transform blockwhen performing intra-prediction, intra-prediction is performed on theprediction block based on a pixel present on the left of the predictionblock, a pixel present at the left top of the prediction block, and apixel present at the top. If the size of a prediction block is differentfrom the size of a transform block and the prediction block includes aplurality of transform blocks when performing intra-prediction, however,intra-prediction may be performed using reference pixels determinedbased on the transform blocks.

Furthermore, one coding block may be partitioned into a plurality ofprediction blocks. If the size of a coding block is a minimum,intra-prediction may be performed using an N×N partition in which onecoding block is partitioned into four prediction blocks having a squareform in relation to only the minimum coding block.

The inter-prediction unit 106 can perform prediction with reference toinformation about a block that is included in at least one picture thatbelong pictures anterior or posterior to a current picture. Theinter-prediction unit 106 may include a reference picture interpolationunit, a motion prediction unit, and a motion compensation unit.

The reference picture interpolation unit can receive information about areference picture from the memory 150 and generate information aboutpixels of an integer pixel or less in the reference picture. In the caseof a luma pixel, a DCT-based 8-tap interpolation filter using differentfilter coefficients may be used in order to generate information aboutpixels of an integer pixel or less in a ¼ pixel unit. In the case of achroma signal, a DCT-based 4-tap interpolation filter using differentfilter coefficients may be used in order to generate information aboutpixels of an integer pixel or less in a ⅛ pixel unit.

The inter-prediction unit 106 can perform motion prediction based on areference picture that has been interpolated by the reference pictureinterpolation unit. Various methods, such as a Full search-based BlockMatching Algorithm (FBMA), Three Step Search (TSS), and a New Three-Stepsearch algorithm (NTS), may be used as a method for calculating a motionvector. The motion vector may have an integer or a motion vector valueof a ½ or ¼ pixel unit based on an interpolated pixel. Theinter-prediction unit 106 can perform prediction on a current blockusing one of multiple inter-prediction methods. Various methods, such asa skip method, a merge method, and an Advanced Motion Vector Prediction(AMVP) method, may be used as the inter-prediction methods.

A residual block including information about a residual value, that is,a difference between a predicted block (i.e., an intra-predicted blockor an inter-predicted block) generated from the prediction unit 110 andthe original block of the predicted block, can be generated.

The residual block can be inputted to the transform unit 115. Thetransform unit 115 can transform the residual block, includinginformation about a residual value between the original block and apredicted block, using a transform method, such as Discrete CosineTransform (DCT) or Discrete Sine Transform (DST). Whether DCT or DSTwill be applied in order to transform the residual block may bedetermined based on information about an intra-prediction mode for aprediction block used to generate the residual block and informationabout the size of the prediction block. That is, the transform unit 115may use a different transform method depending on the size of aprediction block and a prediction method.

The quantization unit 120 can quantize values that have been transformedinto a frequency domain by the transform unit 115. A quantizationcoefficient may vary depending on a block or the importance of an image.A value calculated by the quantization unit 120 can be provided to theinverse quantization unit 135 and the rearrangement unit 125.

The rearrangement unit 125 can perform the rearrangement of coefficientvalues on the quantized residual value. The rearrangement unit 125 canchange coefficients of a two-dimensional block form into aone-dimensional vector form through a coefficient scanning method. Forexample, the rearrangement unit 125 can change coefficients of atwo-dimensional block form into a one-dimensional vector form byperforming scanning from a DC coefficient to coefficients of a highfrequency domain using an up-right diagonal scan method. A vertical scanmethod of scanning coefficients of a two-dimensional block form in acolumn direction or a horizontal scan method of performing scanningcoefficients of a two-dimensional block form in a row direction not theup-right diagonal scan method may be used depending on the size of atransform block and an intra-prediction mode. That is, which one of theup-right diagonal scan method, the vertical scan method, and thehorizontal scan method will be used may be determined depending on thesize of a transform block and an intra-prediction mode.

The entropy coding unit 130 can perform entropy coding based on valuescalculated by the rearrangement unit 125. The entropy coding may beperformed using at least one of various coding methods, such asexponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), andContext-Adaptive Binary Arithmetic Coding (CABAC).

The entropy coding unit 130 can receive various pieces of information,such as information about a residual coefficient of a coding block,information about a block type, information about a prediction mode,information about a partition unit, information about a predictionblock, information about a motion vector, information about a referenceframe, and block interpolation, and filtering information, from therearrangement unit 125 and the prediction unit 110 and perform entropycoding according to a specific coding method using the various pieces ofinformation. Furthermore, the entropy coding unit 130 can performentropy coding on a residual coefficient of a coding block received fromthe rearrangement unit 125.

The inverse quantization unit 135 and the inverse transform unit 140perform inverse quantization on values quantized by the quantizationunit 120 and inverse transform on values transformed by the transformunit 115, respectively. The residual values generated from the inversequantization unit 135 and the inverse transform unit 140 can be combinedwith a prediction block predicted through the motion estimation unit,the motion compensation unit and the intra-prediction unit included inthe prediction unit 110, thereby being capable of generating areconstructed block.

The filter unit 145 may include at least one of a deblocking filter, anoffset correction unit, and an Adaptive Loop Filter (ALF).

The deblocking filter removes block distortion, generated due to theboundary between blocks, from a reconstructed picture. In order todetermine whether or not to perform deblocking, whether or not to applythe deblocking filter to a current block may be determined based onpixels included in some columns or rows of the current block. If thedeblocking filter is applied to the current block, a strong filter or aweak filter may be applied depending on the strength of necessarydeblocking filtering. Furthermore, in applying the deblocking filter,horizontal filtering and vertical filtering may be performed in parallelwhen performing the vertical filtering and the horizontal filtering.

The offset correction unit can correct an image on which deblocking hasbeen performed in a pixel unit using a difference between the image andthe original image as an offset. In order to perform offset correctionon a specific picture, a method of dividing pixels included in an imageinto a specific number of regions, determining a region on which theoffset will be performed, and applying the offset to the determinedregion, or a method of applying an offset by taking information aboutthe edge of each pixel into consideration may be used.

The Adaptive Loop Filter (ALF) can perform filtering based on a value,that is, a result of a comparison between a filtered and reconstructedimage and the original image. Pixels included in an image may be dividedinto one or more groups, one filter to be applied to each group may bedetermined, and different filtering may be performed on each group.Information regarding whether the ALF will be applied or not may betransmitted as a luma signal for each coding block, and the size andcoefficients of an ALF to be applied may be different depending on eachblock. The ALF may have various forms, and the number of coefficientsincluded in an ALF may be different. Filtering-related information(e.g., information about filter coefficients, ALF on/off information,and information about a filter form) about the ALF may be included in aparameter set in a bit stream form and transmitted.

The memory 150 can store a reconstructed block or picture obtained bythe filter unit 145, and the stored reconstructed block or picture canbe provided to the prediction unit 110 when performing inter-prediction.

FIG. 2 is a block diagram of a decoder in accordance with an embodimentof the present invention.

Referring to FIG. 2, the decoder may include an entropy decoding unit210, a rearrangement unit 215, an inverse quantization unit 220, aninverse transform unit 225, a prediction unit 230, a filter unit 235,and memory 240.

When a bit stream is received from the coder, the input bit stream canbe decoded in a procedure opposite the procedure of the coder.

The entropy decoding unit 210 can perform entropy decoding in aprocedure opposite a procedure performed by the entropy coding unit ofthe coder. Information for generating a prediction block, of pieces ofinformation decoded by the entropy decoding unit 210, is provided to theprediction unit 230. Residual values, that is, the results of theentropy decoding performed by the entropy decoding unit 210, can beinputted to the rearrangement unit 215.

The entropy decoding unit 210 can decode information related tointra-prediction and inter-prediction performed by the coder.

The rearrangement unit 215 can perform rearrangement on the bit streamon which entropy decoding has been performed by the entropy decodingunit 210 according to a method rearranged by the coder. Therearrangement unit 215 can rearrange coefficients represented in aone-dimensional vector form by reconstructing the coefficients intocoefficients of a two-dimensional block form. The rearrangement unit 215can receive information related to the scanning of coefficientsperformed by the coder and perform rearrangement on the informationaccording to an inverse scanning method based on scanning sequenceperformed by the coder.

The inverse quantization unit 220 can perform inverse quantization basedon a quantization parameter provided by the coder and the coefficientvalues of a rearranged block.

The inverse transform unit 225 can perform inverse DCT and inverse DSTon the results of quantization performed by the coder in relation to DCTand DST performed by the transform unit of the coder. The inverse DCTand inverse DST may be performed based on a transform block determinedby the coder. The transform unit of the coder may selectively performDCT and DST depending on a plurality of pieces of information, such as aprediction method, the size of a current block, and a predictiondirection. Like in the coder, the inverse transform unit 225 of thedecoder can perform inverse DCT and inverse DST based on transforminformation on which the transform unit has performed DCT and DST. Whenperforming inverse DCT and inverse DST, the inverse transform unit 225of the decoder may perform inverse DCT and inverse DST based on a codingblock not based on a transform block.

The prediction unit 230 can generate a prediction block based oninformation related to the generation of a prediction block that hasbeen provided by the entropy decoding unit 210 and a previously decidedblock provided by the memory 240 or picture information.

Like in an operation performed by the coder as described above, if thesize of a prediction block is the same as the size of a transform blockwhen performing intra-prediction, intra-prediction is performed on aprediction block based on a pixel on the left of the prediction block, apixel at the left top of the prediction block, and a pixel at the top ofthe prediction block. If a transform block is included in a predictionblock when performing intra-prediction, however, intra-prediction may beperformed using a reference pixel based on the transform block.Furthermore, intra-prediction using an N×N partition may be used foronly a coding block having a minimum size as described above.

The prediction unit 230 may include a prediction block determinationunit, an inter-prediction unit, and an intra-prediction unit. Theprediction block determination unit can receive various pieces ofinformation, such as information about the prediction block, informationabout a prediction mode for an intra-prediction method, and informationrelated to the motion prediction of an inter-prediction method, from theentropy decoding unit 210, detect the prediction block in a codingblock, and determine whether inter-prediction or intra-prediction isperformed on the prediction block. The inter-prediction unit can performinter-prediction on a current prediction block based on informationincluded in at least one of pictures anterior or posterior to a currentpicture that includes the current prediction block using informationnecessary to perform the inter-prediction on the current predictionblock which has been provided by the coder.

The inter-prediction unit can determine that a motion prediction methodfor a prediction block included in a corresponding coding block is anyone of a skip mode, a merge mode, and an AMVP mode based on a codingblock in order to perform inter-prediction.

The intra-prediction unit can generate a prediction block based oninformation about pixels within a current picture. If the predictionblock is a prediction block on which intra-prediction has beenperformed, the intra-prediction unit can perform intra-prediction basedon information about an intra-prediction mode for a prediction blockthat has been provided by the coder.

A block predicted by the prediction unit 230 can be merged withinformation about a decoded residual block, thereby being capable ofgenerating a reconstructed block.

The reconstructed block or picture can be provided to the filter unit235. The filter unit 235 may include a deblocking filter, an offsetcorrection unit, and an ALF.

Information regarding whether or not the deblocking filter has beenapplied to a corresponding block or picture and information on whichwhether a strong filter or a weak filter has been applied can bedetermined if the deblocking filter has been applied can be receivedfrom the coder. The deblocking filter of the decoder can receiveinformation related to the deblocking filter from the coder, and thedecoder can perform deblocking filtering on a corresponding block. Likein the coder, first, the decoder performs vertical deblocking filteringand horizontal deblocking filtering, but may perform at least one ofvertical deblocking and horizontal deblocking in an overlapping portion.Vertical deblocking filtering or horizontal deblocking filtering thathas not been previously performed may be performed in the verticaldeblocking filtering and the horizontal deblocking filtering. Such adeblocking filtering process enables the parallel processing ofdeblocking filtering.

The offset correction unit can perform offset correction on areconstructed image based on the type of offset correction that has beenapplied to an image when performing coding and information about anoffset value.

The ALF can perform filtering based on a value, that is, a result of acomparison between the reconstructed image and the original image, afterperforming filtering. The ALF can be applied to a coding block based oninformation regarding whether or not the ALF has been applied that hasbeen provided by the coder and information about ALF coefficients. SuchALF information may be included in a specific parameter set andprovided.

The memory 240 can store the reconstructed picture or block so that thereconstructed picture or block can be used as a reference picture or areference block and provide the reconstructed picture to an output unit.

As described above, although a coding unit is also called a coding blockin embodiments of the present invention, for convenience of description,the coding block may be a block that means a unit on which decoding aswell as coding is performed. Hereinafter, reference pixel filteringmethods in accordance with embodiments of the present inventiondescribed with reference to FIGS. 3 to 7 may be implemented according tothe functions of the unit described with reference to FIGS. 1 and 2, andthe coder and the decoder are included in the scope of the presentinvention.

A transform block used in the following embodiments of the presentinvention may be one block unit of the present invention. That is, atransform block used in the following embodiments of the presentinvention may be interpreted as the meaning of a block, and such anembodiment is also included in the scope of the present invention.

A reference pixel smoothing method in accordance with an embodiment ofthe present invention relates to a method of performing smoothing on areference pixel by adaptively selecting a smoothing method based oninformation about an intra-prediction mode, the size of a current block(or transform block), and information about the size of a predeterminedmaximum transform block in performing smoothing on a reference pixelused to perform intra-prediction.

Subjective picture quality can be improved by adaptively selecting asmoothing method based on information about an intra-prediction mode,the size of a current block (or transform block), and information aboutthe size of a predetermined maximum transform block in performingsmoothing on a reference pixel used to perform intra-prediction andperforming smoothing on a reference pixel using the selected smoothingmethod. Furthermore, a function capable of adaptively selecting asmoothing method based on the size of a maximum transform block and thesize of a transform block can be implemented in an on/off form. If thefunction capable of adaptively selecting a smoothing method based on thesize of a maximum transform block and the size of a transform block isnot used, the coder may not determine the size of a maximum transformblock by taking the smoothing of a reference pixel into considerationwhen determining the size of the maximum transform block.

Furthermore, a function of adaptively selecting a smoothing method basedon information about an intra-prediction mode, the size of a currentblock (or transform block), and information about the size of apredetermined maximum transform block when performing smoothing on areference pixel can be implemented in an on/off form. Accordingly, thecomplexity of the coder and the decoder can be reduced, and coding anddecoding speed can be increased.

FIG. 3 is a conceptual diagram showing a smoothing filter method appliedto a reference pixel in accordance with an embodiment of the presentinvention.

FIG. 3 shows an intra-prediction mode in which smoothing is applied to areference pixel according to the size of a transform block.

FIG. 3(A) shows a case where the size of a transform block is 8×8. Ifthe size of a transform block is 8×8, smoothing can be performed on areference pixel only when the transform block is a specificintra-prediction mode. An intra-prediction mode in which smoothing isperformed on a reference pixel may include a No. 2 intra-prediction mode305, a No. 18 intra-prediction mode 310, and a No. 34 intra-predictionmode 315 that belong to a directional intra-prediction mode and a planarmode that belongs to a non-directional prediction mode.

FIG. 3(B) shows a case where the size of a transform block is 16×16. Ifthe size of a transform block is 16×16, smoothing can be performed on areference pixel only when the transform block is a specificintra-prediction mode. An intra-prediction mode in which smoothing isperformed on a reference pixel may include a No. 2 intra-prediction mode320 to a No. 8 intra-prediction mode 325, a No. 12 intra-prediction mode330 to a No. 24 intra-prediction mode 335, and a No. 28 intra-predictionmode 340 to a No. 34 intra-prediction mode 345 that belong to adirectional intra-prediction mode and a planar mode that belongs to anon-directional prediction mode.

FIG. 3(C) shows a case where the size of a transform block is 32×32. Ifthe size of a transform block is 32×32, smoothing can be performed on areference pixel only when the transform block is a specificintra-prediction mode. An intra-prediction mode in which smoothing isperformed on a reference pixel may include a No. 2 intra-prediction mode350 to the No. 34 intra-prediction mode 355 other than a No. 10intra-prediction mode 360 and a No. 26 intra-prediction mode 365, whichbelong to a directional intra-prediction mode, and a planar mode thatbelongs to a non-directional prediction mode. Referring to FIGS. 3(A) to3(C), the number of intra-prediction modes in which smoothing isperformed on a reference pixel can be increased as the size of atransform block is increased. If such a method is used, a phenomenon inwhich picture quality is deteriorated due to contouring artifacts thatmay occur in a large block relatively more can be prevented.

If the size of a transform block is 4×4, smoothing may not be performedon a reference pixel when performing intra-prediction on the 4×4transform block because whether or not picture quality is deteriorateddoes not greatly depend on whether or not smoothing is performed on areference pixel.

In accordance with an embodiment of the present invention, whether thesize of a transform block is the same as the size of a maximum transformblock or not may be previously determined, and smoothing may be thenperformed on a reference pixel using a different smoothing filter.

For example, the size of a transform block is the same as the size of amaximum transform block, a bi-linear filter may be used as a smoothingfilter. If the size of a transform block is not the same as the size ofa maximum transform block, that is, if the size of a transform block issmaller than the size of a maximum transform block, a smoothing filterhaving filtering coefficients of [1, 2, 1] may be used as a smoothingfilter.

Information about the size of a maximum transform block can betransmitted to the decoder through a higher layer syntax (e.g., aSequence Parameter Set (SPS) or a Picture Parameter Set (PPS)). Theprediction unit of the decoder can determine the type of smoothingfilter to be applied to a current transform block by comparinginformation about the size of a maximum transform block that isindicated by the coder with the size of a transform block on whichintra-prediction is currently performed.

FIG. 4 is a conceptual diagram showing a reference pixel smoothingmethod using a smoothing filter in accordance with an embodiment of thepresent invention.

FIG. 4 shows a smoothing filter that is used when the size of atransform block is not the same as the size of a maximum transformblock.

A filter having filtering coefficients of [1, 2, 1] can be used as thesmoothing filter that is used when the size of a transform block is notthe same as the size of a maximum transform block. The filteringcoefficients are illustrative, and another weight may be used as thefiltering coefficients. Furthermore, the filter having theaforementioned filtering coefficients is a filter into which onlyinformation about only three pixels of a pixel to be filtered and pixelsneighboring the pixel to be filtered has been incorporated. If moreneighboring pixels are used, the number of filter coefficients can beincreased.

If the size of a transform block is nT, a reference pixel may bep[x][y](x=−1, y=−1 . . . nT*2−1, x=−1 . . . nT*2−1, y=−1). If asmoothing filter having coefficients of [1, 2, 1] is used, a filteredreference pixel p[x][y](x=−1, y=−1 . . . nT*2−1, x=−1 . . . nT*2−1,y=−1) can be calculated as in Equation 1 below.pF[−1][nT*2−1]=p[−1][nT*2−1]pF[nT*2−1][−1]=p[nT*2−1][−1]pF[−1][y]=(p[−1][y+1]+2*p[−1][y]+p[−1][y−1]+2)>>2 for y=nT*2−2 . . . 0pF[−1][−1]=(p[−1][0]+2*p[−1][−1]+p[0][−1]+2)>>2pF[x][−1]=(p[x−1][−1]+2*p[x][−1]+p[x+1][−1]+2)>>2 for x=0 . . .nT*2−2  (1)

Referring to Equation 1, for example, if smoothing is performed on apixel X1 410, the pixel X1 410 can be generated as having a pixel valueX1′ that has been filtered by incorporating a pixel value of a leftneighboring pixel X0 400 and a pixel value of a right neighboring pixelX2 420 at a specific ratio.

Particularly, the filtered and calculated pixel value X1′ may be aweighted average calculated by applying weight of 2 to the pixel to befiltered X1 410 and applying weight of 1 to the remaining neighboringpixels X0 400 and X2 420.

In the case of the reference pixel X0 400 at the left top, a bottompixel Y1 430 and the right neighboring pixel X1 410 instead of the leftneighboring pixel may be used as neighboring pixels.

FIG. 5 is a conceptual diagram showing a reference pixel smoothingmethod using a smoothing filter in accordance with an embodiment of thepresent.

FIG. 5 shows a bi-linear smoothing filter (or a bi-linear interpolationfilter) that is used when the size of a transform block is the same asthe size of a maximum transform block.

The bi-linear smoothing filter is a filter that calculates a weightedaverage by incorporating a ratio of the distances of specific pixels andperforms filtering using the weighted average. For example, a pixelvalue can be calculated by performing a weighted average and filteringbased on a ratio of the positions of pixels filtered based on values ofthe first pixel X0 500 and the last pixel value X8 580 that belong topixels to be filtered.

When reference pixels have similar values, if the [1,2,1] filterdescribed with reference to FIG. 4 is used, subjective picture qualityis deteriorated. In such a case, it will be better to use a bi-linearsmoothing filter in terms of subjective picture quality. If the size ofa transform block is the same as the size of a maximum transform block,there is a good possibility that the reference pixels of a block may besimilar pixels. In this case, the reference pixels may be filtered usinga bi-linear smoothing filter.

If the size of a transform block is the same as the size of a maximumtransform block and reference pixels have similar values to some extent,the reference pixels may be filtered using a bi-linear smoothing filter.Whether reference pixels have similar values to what extent may bedetermined according to Equation 2. If all conditions disclosed inEquation 2 are satisfied, it may be determined that reference pixelshave similar values to some extent and thus bi-linear smoothing may beperformed.|p[−1][−1]+p[nT*2−1][−1]−2*p[nT−1][−1]|<(1<<(bitDepthY-5))|p[−1][−1]+p[−1][nT*2−1]−2*p[−1][nT−1]|<(1<<(bitDepthY-5))  (2)

In Equation 2, ‘bitDepthY’ is the number of bits of a pixel. The numberof bits of a pixel is typically 8 bits and may be 10 bits or 12 bitsaccording to circumstances. If the size of a transform block is nT, areference pixel may be p[x][y](x=−1, y=−1 . . . nT*2−1, x=−1 . . .nT*2−1, y=−1).

If a bi-linear smoothing filter is used, a filtered reference pixelp[x][y](x=−1, y=−1 . . . nT*2−1, x=−1 . . . nT*2−1, y=−1) can becalculated as in Equation 3 below.pF[−1][nT*2−1]=p[−1][nT*2−1]pF[nT*2−1][−1]=p[nT*2−1][−1]pF[−1][y]=(nT*2−1−y)*p[−1][−1]+(y+1)*p[−1][nT*2−1]+nT)>>(1+log₂(nT)) fory=0 . . . nT*2−2pF[−1][−1]=p[−1][−1]pF[x][−1]=((nT*2−1−x)*p[−1][−1]+(x+1)*p[nT*2−1][−1]+nT)>>(1+log₃(nT))for x=0 . . . nT*2−2  (3)

In an intra-prediction method in accordance with an embodiment of thepresent invention, intra-prediction can be performed using a differentintra-prediction method even in the same intra-prediction mode based onthe size of a transform block and the size of a maximum transform block.

For example, if an intra-prediction mode for a transform block includesNo. 1 (DC mode), No. 26 (vertical mode), and No. 10 (horizontal mode),the size of the transform block may be compared with the size of amaximum transform block, and a prediction block may be generated usingdifferent methods when the size of the transform block is the same asand not the same as the size of the maximum transform block.

If the size of a transform block is not the same as the size of amaximum transform block, the predicted sample of a prediction blockwhose an intra-prediction mode includes No. 1 (DC mode), No. 26(vertical mode), and No. 10 (horizontal mode) may be generated as inEquation 4 below.

(1) If the intra-prediction mode is No. 1 (DC mode):

$\left. {{{{{{predSample}\lbrack 0\rbrack}\lbrack 0\rbrack} = \left( {{{p\left\lbrack {- 1} \right\rbrack}\lbrack 0\rbrack} + {2^{*}{dcVal}} + {{p\lbrack 0\rbrack}\left\lbrack {- 1} \right\rbrack} + 2} \right)}\operatorname{>>}2}{{{{{{predSample}\lbrack x\rbrack}\lbrack 0\rbrack} = \left( {{{p(x\rbrack}\left\lbrack {- 1} \right\rbrack} + {3^{*}{dcVal}} + 2} \right)}\operatorname{>>}2},{{{with}\mspace{14mu} x} = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{{{{predSample}\lbrack 0\rbrack}\lbrack y\rbrack} = \left( {{{p\left\lbrack {- 1} \right\rbrack}\lbrack y\rbrack} + {3^{*}{dcVal}} + 2} \right)}\operatorname{>>}2},{{{with}\mspace{14mu} y} = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{{{predSample}\lbrack x\rbrack}\lbrack y\rbrack} = {dcVal}},{{with}\mspace{14mu} x},{y = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{dcVal} = \left( {{\sum\limits_{x^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack x^{\prime} \right\rbrack}\left\lbrack {- 1} \right\rbrack}} + {\sum\limits_{y^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack {- 1} \right\rbrack}\left\lbrack y^{\prime} \right\rbrack}} + {nT}} \right)}\;\operatorname{>>}\left( k{+ 1} \right.}} \right),{{{where}\mspace{14mu} k} = {\log_{2}({nT})}}$

(2) If the intra-prediction mode is No. 26 (vertical mode):predSample[x][y]=p[x][−1], with x,y=0 . . . nT−1

(3) If the intra-prediction mode is No. 10 (horizontal mode):predSample[x][y]=p[−1][y], with x,y=0 . . . nT−1  (4)

If the size of a transform block is the same as the size of a maximumtransform block, the predicted sample of a prediction block whoseintra-prediction mode includes No. 1 (DC mode), No. 26 (vertical mode),and No. 10 (horizontal mode) may be generated as Equation 5 below.

(1) If the intra-prediction mode is No. 1 (DC mode):

$\left. {{{{{{predSample}\lbrack x\rbrack}\lbrack y\rbrack} = {dcVal}},{{with}\mspace{14mu} x},{y = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{dcVal} = \left( {{\sum\limits_{x^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack x^{\prime} \right\rbrack}\left\lbrack {- 1} \right\rbrack}} + {\sum\limits_{y^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack {- 1} \right\rbrack}\left\lbrack y^{\prime} \right\rbrack}} + {nT}} \right)}\operatorname{>>}\left( k{+1} \right.}} \right),{{{where}\mspace{14mu} k} = {\log_{2}({nT})}}$

(2) If the intra-prediction mode is No. 26 (vertical mode):predSample[x][y]=p[x][−1], with x=1 . . . NT−1 and y=0 . . . nT−1predSample[x][y]=Clip1Y{p[x][−1]+((p[−1][y]−p[−1][−1])>>1}, with x=0,y=0, . . . nT−1

(3) If the intra-prediction mode is No. 10 (horizontal mode):predSample[x][y]=p[−1][y], with x=0 . . . nT−1 and y=1 . . . nT−1predSample[x][y]=Clip1Y{p[−1][y]+((p[x][−1]−p[−1][−1])>>1}, with x=0 . .. nT−1, y=0  (5)

That is, in an intra-prediction method in accordance with an embodimentof the present invention, a smoothing method and a method of generatinga prediction block according to a specific intra-prediction mode may bedifferent depending on whether or not the size of a transform block isthe same as the size of a maximum transform block.

Information about the smoothing method and the method of generating aprediction block according to a specific intra-prediction mode may benewly defined in a specific syntax element or may be included ininformation about an existing syntax element, coded, and transmitted tothe decoder.

For example, if a flag becomes ‘on’ based on information about the flag,a reference pixel may be smoothed using a bi-linear smoothing method andthe intra-prediction method defined in Equation 5 may be performed. Incontrast, if the flag becomes ‘off’, the reference pixel may be smoothedusing the [1,2,1] filter and the intra-prediction method defined inEquation 4 may be performed. Such a method is described below as anexample.

For example, information about the smoothing method and informationabout the method of generating a prediction block may be separatelycoded. Furthermore, information about the smoothing method andinformation about the method of generating a prediction block may becoded and decoded as a syntax element having a plurality of bits notbeing a flag.

Furthermore, if an existing reference pixel smoothing method and anexisting intra-prediction method are used without change, coding may beperformed to indicate whether or not the existing reference pixelsmoothing method and the existing intra-prediction method will be usedalong with a reference pixel smoothing method and a method of generatinga prediction block disclosed in the present invention or whether or notthe reference pixel smoothing method and the intra-prediction methoddisclosed in the present invention will be used.

Such information about a flag or a syntax element may be defined invarious higher level syntaxes and may be coded and decoded as headerinformation. For example, the flag information may be defined as varioussyntax structures, such as a Video Parameter Syntax (VPS), a SequenceParameter Set (SPS), a Picture Parameter Set (PPS), and a slice segmentheader, and transmitted.

FIG. 6 is a flowchart illustrating a method of coding information fordetermining whether or not to perform smoothing on a reference pixel inaccordance with an embodiment of the present invention.

Referring to FIG. 6, whether or not to use the smoothing method inaccordance with an embodiment of the present invention described withreference to FIGS. 3 to 5 or an existing smoothing method will be usedcan be determined, and the determined result can be coded as flaginformation at step S600.

If only the smoothing method in accordance with an embodiment of thepresent invention described with reference to FIGS. 3 to 5 is usedwithout determining whether or not to use the smoothing method inaccordance with an embodiment of the present invention or the existingsmoothing method, step S600 of coding flag information may not beperformed.

If the smoothing method described with reference to FIGS. 3 to 5 inaccordance with an embodiment of the present invention is used, aprediction block can be generated according to the following procedure.

Whether or not to perform smoothing on a reference pixel is determinedbased on the size of a current transform block and information about anintra-prediction mode for the current transform block at step S610.

As described above with reference to FIG. 3, whether or not to performreference pixel smoothing on the current transform block can bedetermined based on the size of a transform block and information aboutan intra-prediction mode used by the transform block. The size of atransform block and the intra-prediction mode in which reference pixelsmoothing according to the size of the transform block is performed,described with reference to FIG. 3, may be changed. That is, if whetheror not to perform reference pixel smoothing is determined based on thesize of a transform block and an intra-prediction mode used by thetransform block, this may be included in an embodiment of the presentinvention.

A smoothing method is determined by comparing the size of the currenttransform block with the size of a maximum transform block at step S620.

If, as a result of the determination at step S610, it is determined thatsmoothing is performed on the reference pixel of the transform block,what smoothing method will be used can be determined based on the sizeof the current transform block and the size of the maximum transformblock.

As described above with reference to FIGS. 4 and 5, if the size of acurrent transform block is different from the size of a maximumtransform block, smoothing can be performed using a filter havingfiltering coefficients of [1,2,1]. If the size of a current transformblock is the same as the size of a maximum transform block, smoothingcan be performed using the bi-linear filter described with reference toFIG. 5.

Smoothing is performed on a reference pixel at step S630.

The smoothing can be performed based on the smoothing method determinedat step S620.

A prediction block generated based on the reference pixel smoothedthrough step S600 to step S630 can be added to a residual block, storedin the memory as a reconstructed block, and transmitted as informationfor predicting another subsequent block or picture.

FIG. 7 is a flowchart illustrating a method of decoding information fordetermining whether or not to perform smoothing on a reference pixel inaccordance with an embodiment of the present invention.

Referring to FIG. 7, whether or not to use the smoothing method inaccordance with an embodiment of the present invention described withreference to FIGS. 3 to 5 or an existing smoothing method will be usedcan be determined, and the determined result can be decoded as flaginformation at step S700.

If only the smoothing method in accordance with an embodiment of thepresent invention described with reference to FIGS. 3 to 5 is usedwithout determining whether or not to use the smoothing method inaccordance with an embodiment of the present invention or the existingsmoothing method, step S700 of decoding flag information may not beperformed.

If the smoothing method described with reference to FIGS. 3 to 5 inaccordance with an embodiment of the present invention is used, aprediction block can be generated according to the following procedure.

Whether or not to perform smoothing on a reference pixel is determinedbased on the size of a current transform block and information about anintra-prediction mode for the current transform block at step S710.

As described above with reference to FIG. 3, whether or not to performreference pixel smoothing on the current transform block can bedetermined based on the size of a transform block and information aboutan intra-prediction mode used by the transform block. The size of atransform block and the intra-prediction mode in which reference pixelsmoothing according to the size of the transform block is performed,described with reference to FIG. 3, may be changed. That is, if whetheror not to perform reference pixel smoothing is determined based on thesize of a transform block and an intra-prediction mode used by thetransform block, this may be included in an embodiment of the presentinvention.

A smoothing method is determined by comparing the size of the currenttransform block with the size of a maximum transform block at step S720.

If, as a result of the determination at step S710, it is determined thatsmoothing is performed on the reference pixel of the transform block,what smoothing method will be used can be determined based on the sizeof the current transform block and the size of the maximum transformblock.

As described above with reference to FIGS. 4 and 5, if the size of acurrent transform block is different from the size of a maximumtransform block, smoothing can be performed using a filter havingfiltering coefficients of [1,2,1]. If the size of a current transformblock is the same as the size of a maximum transform block, smoothingcan be performed using the bi-linear filter described with reference toFIG. 5.

Smoothing is performed on a reference pixel at step S730.

The smoothing can be performed based on the smoothing method determinedat step S720.

A prediction block generated based on the reference pixel smoothedthrough step S700 to step S730 can be added to a residual block, storedin the memory as a reconstructed block.

Table 1 and Table 2 below illustrate syntax structures when a smoothingmethod in accordance with an embodiment of the present invention isperformed. Table 1 and Table 2 are illustrative, and other syntaxstructures and syntaxes may be used. Such an embodiment is also includedin the scope of the present invention.

TABLE 1 Sequence_parameter_Set { ....strong_intra_smoothing_enabled_flagif(strong_intra_smoothing_enabled_flagmax_transform_block_size_based_strong_intra_smoothing_flag ....

In Table 1, strong_intra_smoothing_enabled_flag is a flag indicatingwhether or not to use a bi-linear smoothing filter. If the flag is 0,the bi-linear smoothing filter may not be used. If the flag is 1, thebi-linear smoothing filter may be used. However, although the flag is 1,a value of max_transform_block_size_based_strong_intra_smoothing_flagmay be additionally transmitted so that how the bi-linear smoothingfilter will be applied can be determined. That is, if a value ofmax_transform_block_size_based_strong_intra_smoothing_flag is 1, whetheror not to use the smoothing method described with reference to FIGS. 3to 5 in accordance with an embodiment of the present invention or anexisting smoothing method may be determined by comparing the size of acurrent transform block with the size of a maximum transform block.Furthermore, if a value ofmax_transform_block_size_based_strong_intra_smoothing_flag is 0, whetheror not to use the smoothing method described with reference to FIGS. 3to 5 in accordance with an embodiment of the present invention or anexisting smoothing method may be determined by comparing the size of acurrent transform block with the size 32 of a block (i.e., a 32×32 blocksize).

strong_intra_smoothing_enabled_flag andmax_transform_block_size_based_strong_intra_smoothing_flag shown inTable 1 do not need to be necessarily placed in Sequence_parameter_Set.In another embodiment, strong_intra_smoothing_enabled_flag andmax_transform_block_size_based_strong_intra_smoothing_flag shown inTable 1 may be placed in a Video Parameter Set (VPS) or a PictureParameter Set (PPS). Furthermore, strong_intra_smoothing_enabled_flagand max_transform_block_size_based_strong_intra_smoothing_flag do notneed to be placed in the same header information. That is, in yetanother embodiment, strong_intra_smoothing_enabled_flag may be placed inSequence_parameter_Set (SPS), andmax_transform_block_size_based_strong_intra_smoothing_flag may be placedin the PPS under the SPS.

Furthermore, information about a flag indicating whether or not to usethe method of generating a prediction block described in connection withEquation 4 and Equation 5 may be transferred as in Table 2.

TABLE 2 Sequence_parameter_Set { ....max_transform_block_size_based_intra_smoothing_ restriction_flag ....

That is, in Table 2, if a value ofmax_transform_block_size_based_intra_smoothing_restriction_flag is 1,the size of a transform block can be compared with the size of a maximumtransform block as described in connection with Equation 4 and Equation5 and a prediction block can be generated using Equation 5 or Equation5.

If a value ofmax_transform_block_size_based_intra_smoothing_restriction_flag is 0,intra-prediction can be performed according to an intra-prediction modebased on Equation 6 and Equation 7.

If a value ofmax_transform_block_size_based_intra_smoothing_restriction_flag is 0 andthe size of a transform block is smaller than 32×32, a prediction blockcan be generated using Equation 6.

(1) If the intra-prediction mode is No. 1:

$\left. {{{{{{predSample}\lbrack 0\rbrack}\lbrack 0\rbrack} = \left( {{{p\left\lbrack {- 1} \right\rbrack}\lbrack 0\rbrack} + {2^{*}{dcVal}} + {{p\lbrack 0\rbrack}\left\lbrack {- 1} \right\rbrack} + 2} \right)}\operatorname{>>}2}{{{{{{predSample}\lbrack x\rbrack}\lbrack 0\rbrack} = \left( {{{p(x\rbrack}\left\lbrack {- 1} \right\rbrack} + {3^{*}{dcVal}} + 2} \right)}\operatorname{>>}2},{{{with}\mspace{14mu} x} = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{{{{predSample}\lbrack 0\rbrack}\lbrack y\rbrack} = \left( {{{p\left\lbrack {- 1} \right\rbrack}\lbrack y\rbrack} + {3^{*}{dcVal}} + 2} \right)}\operatorname{>>}2},{{{with}\mspace{14mu} y} = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{{{predSample}\lbrack x\rbrack}\lbrack y\rbrack} = {dcVal}},{{with}\mspace{14mu} x},{y = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{dcVal} = \left( {{\sum\limits_{x^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack x^{\prime} \right\rbrack}\left\lbrack {- 1} \right\rbrack}} + {\sum\limits_{y^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack {- 1} \right\rbrack}\left\lbrack y^{\prime} \right\rbrack}} + {nT}} \right)}\;\operatorname{>>}\left( k{+ 1} \right.}} \right),{{{where}\mspace{14mu} k} = {\log_{2}({nT})}}$

(2) If the intra-prediction mode is No. 26:predSample[x][y]=p[x][−1], with x,y=0 . . . nT−1

(3) the intra-prediction mode is No. 10:predSample[x][y]=p[−1][y], with x,y=0 . . . nT−1  (6)

Furthermore, if a value ofamax_transform_block_size_based_intra_smoothing_restriction_flag is 0and the size of a transform block is 32×32, a prediction block can begenerated using Equation 7. Since the size of the transform block is32×32, ‘nT’ becomes 32 in Equation 7.

(1) If the intra-prediction mode is No. 1:

$\left. {{{{{{predSample}\lbrack x\rbrack}\lbrack y\rbrack} = {dcVal}},{{with}\mspace{14mu} x},{y = {{1\mspace{14mu}\ldots\mspace{14mu}{nT}} - 1}}}{{{dcVal} = \left( {{\sum\limits_{x^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack x^{\prime} \right\rbrack}\left\lbrack {- 1} \right\rbrack}} + {\sum\limits_{y^{\prime} = 0}^{{nT} - 1}{{p\left\lbrack {- 1} \right\rbrack}\left\lbrack y^{\prime} \right\rbrack}} + {nT}} \right)}\operatorname{>>}\left( k{+1} \right.}} \right),{{{where}\mspace{14mu} k} = {\log_{2}({nT})}}$

(2) If the intra-prediction mode is No. 26:predSample[x][y]=p[x][−1], with x=1 . . . nT−1 and y=0 . . . nT−1predSample[x][y]=Clip1Y{p[x][−1]+((p[−1][y]−p[−1][−1])>>1}, with x=0,y=0, . . . nT−1

(3) If the intra-prediction mode is No. 10:predSample[x][y]=p[−1][y], with x=0 . . . nT−1 and y=1 . . . nT−1predSample[x][y]=Clip1Y{p[−1][y]+((p[x][−1]−p[−1][−1])>>1}, with x=0 . .. nT−1, y=0  (7)

max_transform_block_size_based_intra_smoothing_restriction_flag shown inTable 2 does not need to be necessarily placed inSequence_parameter_Set. In another embodiment,max_transform_block_size_based_intra_smoothing_restriction_flag may beplaced in a Video Parameter Set (VPS) or a Picture Parameter Set (PPS).

Furthermore,max_transform_block_size_based_intra_smoothing_restriction_flag may beplaced in the same header information as that ofstrong_intra_smoothing_enabled_flag and/ormax_transform_block_size_based_strong_intra_smoothing_flag using Table 1or may be placed in header information different from that ofstrong_intra_smoothing_enabled_flag and/ormax_transform_block_size_based_strong_intra_smoothing_flag shown inTable 1.

While the invention has been shown and described with respect to thepreferred embodiments, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

What is claimed is:
 1. An intra-prediction method in which anintra-prediction is performed on a current block, comprising:determining whether or not to perform smoothing on a reference pixel ofthe current block based on a size of a block and an intra-predictionmode for the current block, the size of the block being indicated by anencoder and transmitted to a decoder through a higher layer syntax;determining a smoothing filter by comparing the size of the currentblock with the transmitted size of the block, and in response to theresult of comparison between the size of the current block and thetransmitted size of the block, determining the smoothing filter to be abi-linear smoothing filter or a smoothing filter having predeterminedfiltering coefficients.
 2. An intra-prediction method in which anintra-prediction is performed on a current block, comprising:determining whether or not to perform smoothing on a reference pixel ofthe current block based on a size of a block and an intra-predictionmode for the current block, the size of the block being indicated by anencoder and transmitted to a decoder through a higher layer syntax; anddetermining a smoothing method by comparing the size of the currentblock with the transmitted size of the block, wherein the determining ofwhether or not to perform smoothing on the reference pixel based on thetransmitted size of the block and the intra-prediction mode for thecurrent block comprises determining to perform smoothing only inresponse to each of: the size of the current block being 8×8 and theintra-prediction mode for the current block being a No. 2intra-prediction mode, the size of the current block being 8×8 and theintra-prediction mode for the current block being a No. 18intra-prediction mode, the size of the current block being 8×8 and theintra-prediction mode for the current block being a No. 34intra-prediction mode, the size of the current block being 8×8 and theintra-prediction mode for the current block being a planar mode; thesize of the current block being 16×16 and the intra-prediction mode forthe current block being a No. 2 intra-prediction mode to a No. 8intra-prediction mode, the size of the current block being 16×16 and theintra-prediction mode for the current block being a No. 12intra-prediction mode to a No. 24 intra-prediction mode, the size of thecurrent block being 16×16 and the intra-prediction mode for the currentblock being a No. 28 intra-prediction mode to a No. 34 intra-predictionmode, and the size of the current block being 16×16 and theintra-prediction mode for the current block being a planar mode; and thesize of the current block being 32×32 and the intra-prediction mode forthe current block being a No. 2 intra-prediction mode to a No. 34intra-prediction mode, and the size of the current block being 32×32 andthe intra-prediction mode for the current block being a planar modeother than a No. 10 intra-prediction mode and a No. 26 intra-predictionmode.
 3. The intra-prediction method of claim 1, wherein the smoothingfilter having the filtering coefficients of [1,2,1] is a filter forcalculating a filtered reference pixel,pF[x][y](x=−1,y=−1 . . . nT*2-1,x=−1 . . . nT*2-1,y=−1), according tothe equations:pF[−1][nT*2−1]=p[−1][nT*2−1]pF[nT*2−1][−1]=p[nT*2−1][−1]pF[−1][y]=(p[−1][y+1]+2*p[−1][y]+p[−1][y−1]+2)>>2 for y=nT*2−2 . . . 0pF[−1][−1]=(p[−1][0]+2*p[−1][−1]+p[0][−1]+2)>>2pF[x][−1]=(p[x−1][−1]+2*p[x][−1]+p[x+1][−1]+2)>>2 for x=0 . . . nT*2−2wherein the current block is a current transform block and the size ofthe current block is a size of a current transform block, and whereinthe size of the current transform block is nT, a reference pixel of thecurrent transform block isp[x][y](x=−1,y=−1 . . . nT*2−1,x=−1 . . . nT*2−1,y=1).
 4. Theintra-prediction method of claim 1, wherein the bi-linear smoothingfilter is a filter for calculating a filtered reference pixelpF[x][y](x=−1,y=−1 . . . nT*2−1,x=−1 . . . nT*2−1,y=−1). according tothe equations:pF[−1][nT*2−1]=p[−1][nT*2−1]pF[nT*2−1][−1]=p[nT*2−1][−1]pF[−1][y]=(nT*2−1−y)*p[−1][−1]+(y+1)*p[−1][nT*2−1]+nT)>>(1+log₂(nT)) fory=0 . . . nT*2−2pF[−1][−1]=p[−1][−1]pF[x][−1]=((nT*2−1−x)*p[−1][−1]+(x+1)*p[nT*2−1][−1]+nT)>>(1+log₂(nT))for x=0 . . . nT*2−2 wherein the current block is a current transformblock and the size of the current block is a size of a current transformblock, and wherein the size of the current transform block is nT, and areference pixel of the current transform block isp[x][y](x=−1,y=−1 . . . nT*2−1,x=−1 . . . nT*2−1,y=1).
 5. Theintra-prediction method of claim 1, further comprising decoding flaginformation, wherein the flag information is information for determiningwhether or not to perform smoothing on the reference pixel bydetermining whether or not to perform smoothing on the reference pixelbased on the size of the current block and the intra-prediction mode forthe current block and comparing the size of the current block with thesize of the block.
 6. The intra-prediction method of claim 1, furthercomprising determining an intra-prediction method for a specificintra-prediction mode by comparing the size of the current block withthe transmitted size of the block.
 7. An image decoder in which anintra-prediction is performed on a current block, comprising: anintra-prediction unit configured to: determine whether or not to performsmoothing on a reference pixel of the current block based on a size of ablock and an intra-prediction mode for the current block, the size ofthe block being indicated by an encoder and transmitted to the decoderthrough a higher layer syntax, determine a smoothing method by comparingthe size of the current block with the transmitted size of the block, inresponse to the result of comparison between the size of the currentblock and the transmitted size of the block, determine the smoothingfilter to be a bi-linear smoothing filter or a smoothing filter havingpredetermined filtering coefficients.
 8. The image decoder of claim 7,wherein the smoothing filter having the filtering coefficients of[1,2,1] is a filter for calculating a filtered reference pixel,pF[x][y](x=−1,y=−1 . . . nT*2−1,x=−1 . . . nT*2−1,y=1), according to:pF[−1][nT*2−1]=p[−1][nT*2−1]pF[nT*2−1][−1]=p[nT*2−1][−1]pF[−1][y]=(p[−1][y+1]+2*p[−1][y]+p[−1][y−1]+2)>>2 for y=nT*2−2 . . . 0pF[−1][−1]=(p[−1][0]+2*p[−1][−1]+p[0][−1]+2)>>2pF[x][−1]=(p[x−1][−1]+2*p[x][−1]+p[x+1][−1]+2)>>2 for x=0 . . . nT*2−2wherein the current block is a current transform block and the size ofthe current block is a size of a current transform block, and whereinthe size of the transform current block is nT, and a reference pixel ofthe transform block isp[x][y](x=−1,y=−1 . . . nT*2−1,x=−1 . . . nT*2−1,y=−1).
 9. The imagedecoder of claim 7, wherein the bi-linear smoothing filter is a filterfor calculating a filtered reference pixel, pF[x][y] (x=−1, y=−1 . . .nT*2−1, x=−1 . . . nT*2−1, y=−1), according to:pF[−1][nT*2−1]=p[−1][nT*2−1]pF[nT*2−1][−1]=p[nT*2−1][−1]pF[−1][y]=(nT*2−1−y)*p[−1][−1]+(y+1)*p[−1][nT*2−1]+nT)>>(1+log₁(nT)) fory=0 . . . nT*2−2pF[−1][−1]=p[−1][−1]pF[x][−1]=((nT*2−1−x)*p[−1][−1]+(x+1)*p[nT*2−1][−1]+nT)>>(1+log₂(nT))for x=0 . . . nT*2−2 wherein the current block is a current transformblock and the size of the current block is a size of a current transformblock, and wherein the size of the transform block is nT, and areference pixel of the transform block isp[x][y](x=−1,y=−1 . . . nT*2−1,x=−1 . . . nT*2−1,y=−1).
 10. The imagedecoder of claim 7, further comprising an entropy decoding unit, whereinthe entropy decoding unit is configured to decode flag information, andthe flag information is information for determining whether or not toperform smoothing on the reference pixel by determining whether or notto perform smoothing on the reference pixel based on the size of thecurrent block and the intra-prediction mode for the current block andcomparing the size of the current block with the transmitted size of theblock.
 11. The image decoder of claim 7, wherein the intra-predictionunit is configured to determine an intra-prediction method for aspecific intra-prediction mode by comparing the size of the currentblock with the transmitted size of the block.